Connection admission control (CAC) is an important element of ATM traffic management. CAC provides access control by regulating the number and types of connections that can be allowed at any given time for a given amount of resources. In an ATM multi-service network, the resource demand of each connection has to be estimated as a function of the cell-level traffic descriptors, the required quality-of-service (QOS), the states of the network resources, the traffic-stream class or priority, and arguably the nodal architecture. The routing decisions, in turn, depend on the estimated resource demand.
Much of the extensive work on ATM communications focused on the analysis of the aperiodic multiplexer (often called the statistical multiplexer). The main objective of the analysis is to determine performance indices, such as the cell-loss probability and cell-delay, given the storage capacity of the multiplexing facility and the traffic descriptors of the mixture of traffic streams. The equivalent-bit-rate (EBR) of a connection is a natural by-product of the performance analysis. The problem here is to find a nominal bit-rate, the EBR, for each connection so that the system meets the specified performance objectives as long as the sum of the EBR values of the accepted connections does not exceed the capacity of the designated link.
There are various works on calculating bandwidth requirements and the following are some examples.
1! R. J. Gibbens and P. J. Hunt, "Effective Bandwidths for the Multi-Type UAS Channel", Queueing Systems No. 9, (1991) pp. 17-28;
2! G. L. Choudhury, D. M. Lucantoni and Ward Witt, "Squeezing the Most Out of ATM", IEEE Transactions on Communications, Vol. 44, No. 2, February 1996, pp. 203-217;
3! E. Buffet and N. G. Duffield, "Exponential Upper Bounds Via Martingales for Multiplexers with Markovian Arrivals", Journal of Applied Probability (1993);
4! A variation of 1! above, "Equivalent Capacity and Its Application to Bandwidth Allocation in High-Speed Networks" by Guerin et al., IEEE Journal on Selected Areas in Communications, Vol. 9, No. 7, September 1991;
5! The gist of the invention is contained in papers presented by J. Yan and the inventor at Globecom, Singapore, November, 1995, and at the 9th ITC Specialists Seminar, Leidschendam, the Netherlands, November 1995. The papers are entitled "Designing an ATM-Based Broadband Network: An Overview" and "Traffic Considerations in the Synthesis of an ATM-Based Network".
ATM admission control can be based on either of two approaches: a direct performance-evaluation approach or an inverse resource-requirement-estimation approach. In the direct approach, the estimated cell-level performance resulting from the admission of a new connection (or call) is calculated. In the inverse approach, an EBR ("equivalent bit rate", often called the "equivalent bandwidth" or "effective bandwidth") of the new arrival is determined by some artifice or another. The connection is accepted if the remaining unassigned capacity of the route is not less than the calculated EBR. The EBR for a connection which traverses several links may vary from link to link and would be based on the source's traffic descriptors, the cell-level performance objectives, the speed of the link under consideration, and the buffer size. The burst length is usually assumed to be exponentially distributed. The assigned fraction of the capacity of a link, i.e., the sum of the EBR values of all connections traversing the link, divided by the total link capacity, is hereinafter called the call-level occupancy. The actual link utilization is called the cell-level occupancy. The call-level occupancy is generally higher than the cell-level occupancy. It should be noted that the direct and inverse approaches are two aspects of the same technique, in other words, the QOS and EBR are two parameters of the same equation.
The traffic may comprise protected and unprotected streams. The protected streams, such as the CBR (constant-bit-rate) and the VBR (variable-bit-rate) traffic, are promised a specified quality-of-service (QOS). The unprotected streams, such as the UBR (unspecified-bit-rate) traffic, must rely on the unused time slots of the protected streams.
The EBR calculation can be based on several rate-control disciplines such as the per-connection control proposed by Gibbens and Hunt, a class-based control, a link-based control, or any combination of these. These controls and the resulting bandwidth utilization are graphically shown in the following figures.
The per-connection calculation is shown in FIG. 1. There is a mixture of different types of connections, e.g., VBR (variable bit rate) and CBR (constant bit rate). Each connection, a-j, is guaranteed a service rate and the combined service rate for all the connections is the link capacity R. The total mean rate of all the connections is indicated by M, M&lt;R. The EBR for each connection is calculated according to a single-source model, e.g., the Gibbens-Hunt (GH) method referenced 1! above. The Gibbens-Hunt model assumes buffer sharing, but the link capacity is assumed to be divided into partitions each of which is dedicated to a single connection. If link sharing was permitted, the sum E.sub.1 of the EBR values would be less than R. The traffic intensity is also indicated in the figure in relation to time. As there is no sharing of the link capacity among the connections, this control discipline results in a large unused (unassigned) capacity. In practice, the link is actually shared.
Similarly the per-class calculation is shown in FIG. 2. Connections a-j are grouped into classes 0-2. Each class is guaranteed a prescribed service rate. Within a class, the capacity is shared by its connections, e.g., a-d of class 0, resulting in more efficient resource management. The invention modifies the Gibbens-Hunt calculation of the EBR of each connection within a class by taking the capacity sharing into account. This modified method is the technique of the invention. The sum of all the combined EBR values of all the classes is shown as E.sub.2. (Note that E.sub.2 .gtoreq.E.sub.1.) This control realizes a reduction in the unassigned capacity over the per-connection control as the result of sharing gain because the remaining part of the link capacity can be declared as "guaranteed available rate" for new connections. The unassigned capacity is treated in the same way as in the case of per-connection control. There is, however, no sharing among the classes.
In FIG. 3, the per-link control is illustrated. The link capacity R is shared by all its connections, each of which is guaranteed a prescribed service quality. The combined EBR values of all the connections is calculated according to the modified Gibbens-Hunt method of the invention based on the link rate instead of the class allocated rate and is shown as E.sub.3 (E.sub.3 =E.sub.1). The remaining part of the link capacity can now be declared as "guaranteed available rate" and can be offered to new connections of protected traffic.
The part marked scrap in FIGS. 1-3 cannot be used reliably by new connections of protected traffic. In any case, the unused cell time can always be made available to the unprotected traffic streams (such as UBR).
The present invention therefore resides in the bandwidth estimation techniques based on the work described by Gibbens-Hunt in their above-referenced article 1!. It also resides in the field of connection admission control for ATM networks using such an estimated bandwidth.